1. Field of the Invention
This invention generally relates to a light scanning apparatus for causing a light beam to scan along a predetermined path, and, in particular, to a light scanning apparatus employing a semiconductor laser for generating a scanning light beam.
2. Description of the Prior Art
Recently, the semiconductor laser has attracted attention as an information processing light source which is compact in size and low in cost; however, one of the problems residing in the characteristics of the laser is that the oscillation wavelength varies depending upon the temperature. This is because, the energy band gap of the activating layer varies as the temperature changes. One such example is shown in FIG. 1, in which the abscissa is taken for the temperature of a heat sink on which a semiconductor laser is mounted and the ordinate is taken for the oscillation wavelength of the laser output. As shown, as the temperature increases, the oscillation wavelength gradually shifts toward a longer wavelength region, and when the temperature has increased beyond a certain level, the oscillation wavelength abruptly jumps to a larger level as indicated by the dashed line. This phenomenon may be explained such that the oscillation wavelength gradually shifts toward a larger wavelength region as the energy band gap of the activating layer becomes smaller due to an increase in temperature, and when such a gradual change in oscillation wavelength has reached a certain level, the longitudinal mode of oscillation changes to the next mode of oscillation. Typically, such a gradual change in oscillation wavelength occurs at the rate of approximately 0.05-0.07 nm/.degree.C.; on the other hand, a jump in oscillation wavelength is in the order of approximately 0.3 nm.
When a semiconductor laser having such a temperature dependent characteristic is employed as a light source of a light scanning apparatus including a diffraction type light deflecting element such as a hologram light deflector, there will be created a problem of fluctuations in pitch due to temperature variations. That is, changes in the oscillation wavelength of a light beam from a semiconductor laser cause changes in the diffraction angle, and, as a result, the beam spot on the scanning surface will be shifted in position in relation thereto. FIG. 2 illustrates a schematic diagram of a light scanning apparatus which is useful for explaining the above-described problem. The apparatus of FIG. 2 includes a hologram disk 1 fixedly mounted on the shaft of a motor M and serving as a light beam deflector. As shown in FIG. 3, the hologram disk 1 is comprised of a support disk and a plurality of equidistant linear gratings 1a arranged circularly around the rotating axis of and on the flat surface of the disk. Thus, the hologram disk 1 is driven to rotate in a predetermined direction at constant speed during operation. The light emitted from a semiconductor laser light source 2 is shaped into a parallel light beam as passing through a lens 3, which in turn is incident on the hologram disk 1 at an angle .theta..sub.i. The diffracted light beam coming out of the hologram disk 1 at an angle of .theta..sub.d is reflected by a mirror 4 and focused onto a point P.sub.0 on a scanning surface 6 by means of a lens 5.
As well known in the art, the relation between the angles .theta..sub.i and .theta..sub.d may be expressed in the following manner. EQU sin .theta..sub.d =.lambda./d-sin .theta..sub.i ( 1)
where d indicates the pitch of a hologram grating and .lambda. indicates the wavelength of laser light.
When the oscillation wavelength .lambda. of the laser varies due to a change in temperature, the diffraction angle .theta..sub.d changes, so that, as shown in FIG. 2 by the dotted line, the beam spot on the scanning surface 6 slightly shifts its position from point P.sub.0 to point P.sub.1. Such a shift in position by the amount of .DELTA.P will produce a fluctuation in pitch of the scanning line.
From the above equation (1), we can derive the relation between a change in wavelength .DELTA..lambda. and a change in diffraction angle .DELTA..theta..sub.d as follows: EQU .DELTA..theta..sub.d .congruent..DELTA..lambda./(d cos .theta..sub.d) (2)
On the other hand, use is often made of the so-called f.theta. lens as the convergent lens 5, and, with f indicating its focusing distance, the shift amount in position .DELTA.P of the beam spot or light convergent point on the scanning surface 6 may be expressed in the following manner. EQU .DELTA.P=f.DELTA..theta..sub.d (f.DELTA..lambda.)/(d cos .theta..sub.d) (3)
It is obvious from the above equation (3) that the amount of shift in position .DELTA.P is proportional to the amount of variation in wavelength .DELTA..lambda..
For this reason, in order to make the amount of shift in scanning position in the structure of FIG. 2 to be minimum, the amount of variation in wavelength of the light from the semiconductor laser 2 must be made as small as possible, which in turn requires to prevent the semiconductor laser 2 from changing its temperature. For example, if it is desired to limit the amount of shift in beam spot position not to exceed 10 microns under the conditions of f=300 mm, d=0.55 microns and .theta..sub.d =45.degree., the amount of variation in wavelength must be maintained at 0.01 nm or less as calculated from the above equation (3). This implies that the temperature variation of the semiconductor laser 2 must be maintained at approximately 0.1.degree. C. or less, which is practically very difficult to do even if use is made of a precise temperature control means such as an electronic cooling element. Thus, the above-described problem has constituted one of the obstacles in fabricating a practical hologram light scanning apparatus employing a semiconductor laser as its light source.